SELF-ASSEMBLED QUANTUM DOTS FOR OPTOELECTRONIC DEVICES: PROGRESS AND CHALLENGES M.Henini School of Physics and Astronomy, University of Nottingham, Nottingham NG7 2RD, U.K. Tel/Fax: +44 115 9515195/9515180 e-mail: Mohamed.Henini@Nottingham.ac.uk ABSTRACT Low dimensional structures (LDS) form a major new branch of physics research. They are semiconductor structures, which have such a small scale in one or two spatial dimensions that their electronic properties are significantly different from the same material in bulk form. These properties are changed by quantum effects. Throughout the world there is increasing interest in the preparation, study and application of LDS. Their investigation has revitalised condensed matter science, in particular semiconductor materials. These complex LDS offer device engineers new design opportunities for tailor-made new generation electronic and photonic devices. New crystal growth techniques such as molecular beam epitaxy (MBE) and metal-organic chemical vapour (MOCVD) deposition have made it possible to produce such LDS in practice. These sophisticated technologies for the growth of high quality epitaxial layers of compound semiconductor materials on single crystal semiconductor substrates are becoming increasingly important for the development of the semiconductor electronics industry. This article is intended to convey the flavour of the subject by focussing on the technology and applications of self-assembled quantum dots and to give an elementary introduction to some of the essential characteristics. 1. INTRODUCTION Crystal growth and post-growth processing technologies have developed to the extent that it has become possible to fabricate semiconductor structures whose dimensions are comparable with interatomic distances in solids. These structures are known as LDS. The movement of charge carriers in these structures are constrained by potential barriers. This results in the restriction of the degrees of freedom for motion to two, one or even zero. The system becomes two, one or zero dimensional depending on whether the potential barriers confine the carriers in one (layers), two (wires) or three (dots) dimensions (Figure 1), respectively. Carriers exhibit wave-like characteristics and when the layer thickness is comparable with the carrier wavelength the carrier motion is constrained and exciting new physical properties result. - 1 -
The study of LDS began in the late 1970's when sufficiently thin epitaxial layers were first produced. The layers used in the early investigations became available following developments in the technology of the epitaxial growth of semiconductors, mainly pioneered in industrial laboratories for device purposes. One of the main directions of contemporary semiconductor physics is the production and study of structures with a dimension less than two: quantum wires and quantum dots, in order to realize novel devices that make use of low-dimensional confinement effects. During the last few years much attention has been devoted to the strain in the grown layer and characterization of self-assembled semiconductor quantum dots (QDs). The strong interest in these semiconductor nanostructures is motivated by the possibility to use them as active media in future high-speed electronic and photonic devices. FABRICATION METHODS OF QUANTUM DOTS Several methods for the fabrication of QDs have been reported over the last decade including lithography-based technologies. Although this technique is widely used to provide QD predominantly by the combination of high-resolution electron beam lithography and etching, the spatial resolution required for reaching the size regime where significant quantization effects can be expected tends to be larger than the desirable level. In addition, lithographic methods and subsequent processings often produce contamination, defect formation, size non-uniformity, poor interface quality, and even damage to the bulk of the crystal itself. A new attractive method of defect free 10nm scale QD fabrication is the Stranski-Krastanov (SK) growth in lattice-mismatched systems. In the SK growth mode the mismatched epitaxy is initially accommodated by biaxial compression in a layer-by-layer (2D) growth region, traditionally called the wetting layer. After deposition of a few monolayers the strain energy increases and the development of islands (3D) becomes more favourable than planar growth [1] (Figure 2). In the III-V semiconductor material system, SK growth has been used to grow InAs islands on GaAs and it has been shown that the size fluctuation of dots is relatively small ( 10%) and the small dots and surrounding host matrix are dislocation-free and strained coherently with GaAs. It has been reported that the InAs growth mode changes from 2D to 3D upon the deposition of less than 2 monolayers of InAs, so as to reduce the strain in grown layer, since there is about a 7% lattice mismatch in the GaAs/InAs system. The strained (In,Ga)As/GaAs material system has been the most widely studied for which various quantum effects have been demonstrated. Various combinations of III-V semiconductors based on phosphorus or antimony compounds, and Si/SiGe alloys have also been studied. The advantages of this technique of QD fabrication are that no nanotechnology and no further etch or implantation induced process are necessary. Since the dots are grown in-situ a homogeneous surface morphology is maintained and defect creation is avoided. However, the inherent problem associated with this method is the size non-uniformity and the position uncontrollability of the QD. Controlling the dimension and arrangement of the self-organized 3D structures is thought to be very important for obtaining good properties of the structures. The islands become technologically more interesting if it is possible to manipulate their arrangement laterally and vertically (Figure 3) in order to achieve the three-dimensional arrays. There are already several reports on spontaneous lateral ordering due to the preferential nucleation along surface steps. N.Kitamura et al [2] demonstrated successful alignment of InGaAs by using a 2 o off (100) GaAs substrate with multi-atomic steps in MOCVD growth process. Pre-patterned substrates have also been used for ordering of QDs in a more direct way. D.S.L.Miu et al [3] grew by MBE on etched GaAs gratings and found islands to form on the sidewalls of ridges running along [1-10] direction. Similar results were obtained by S.Jeppesen et al [4] for Chemical Beam Epitaxy (CBE) deposited InAs islands in wet-etched and partially overgrown trenches and holes on a (100) GaAs surface. They formed chains of InAs islands aligned in trenches along [011]. The chains of islands have 33nm minimum periods. The vertical alignment is expected and the total density can be increased by stacking the QDs with a spacer layer. Vertically aligned and electronically coupled islands has several advantages including the application of the tunnelling process to novel electronic devices such as single electron tunnelling devices, the study of tunnelling dynamics between QDs, and the high QD density for QD lasers. Several groups have recently successfully grown stacked InAs self-assembled QD structures separated by GaAs spacer layers by MBE. Y.Sugiyama et al [5] reported vertically aligned InAs QDS up the ninth layer with - 2 -
2.5nm monolayers InAs and 1.5nm GaAs spacer layers. G.S.Solomon et al [6] demonstrated arrays of InAs islands which are vertically stacked, vertically aligned and electronically coupled in the growth direction. They have achieved vertical alignment of up to 10 islanding layers with no associated dislocation generation. PHOTOLUMINESCENCE PROPERTIES OF InAs/GaAs QUANTUM DOTS In this section I will report on the photoluminescence properties of multiple (InGa)As/(AlGa)As QD layers grown by molecular beam epitaxy under different conditions (i.e., different Al content, number of QD layers, and different spacer thickness between QD layers). We found that by varying the Al content in the (AlGa)As matrix and/or stacking several QD layers, the room temperature dot luminescence is tuned over a wavelength range from 0.8 µm to 1.3 µm. Three different sets of samples were considered. In the first set, three InAs layers were embedded in an Al y Ga 1-y As matrix grown at T G =520 ºC. The average thickness, L, of each InAs layer is 1.8 monolayer, ML. The three InAs layers are separated from each other by 20 nm-thick Al y Ga 1-y As barriers (y=(0.0-0.8)), resulting in uncoupled dots. In the second set, 1 and 10 layers of InAs dots (L=1.8 ML) were embedded in a GaAs matrix. The vertically stacked InAs layers were separated from each other by a distance d = 1.7 nm. This structure was grown at T G = 500 ºC. Finally, in the third set of samples, three InAs layers were embedded each of them in a GaAs/Al 0.3 Ga 0.7 As quantum well and grown at T G =500 ºC. The dot formation was controlled in situ by monitoring the reflection high-energy electron-diffraction pattern. Photoluminescence (PL) measurements were performed from T=10 K to T ~ 500 K. The optical excitation was provided by the 514.5 nm line of an Ar + laser. The luminescence was dispersed by a 3/4 m monochromator and detected by a cooled Ge diode. Figure 4 shows the room temperature QD PL emission for three representatives InAs QD samples. The three samples have different Al content in the Al y Ga 1-y As matrix surrounding the dots or have different spacing (d) or number (N) of the InAs QD layers. With increasing y, the QD PL band blue-shifts from 1.1 µm (sample A: y = 0, d = 20 nm, N = 3) to 0.8 µm (sample B: y = 0.8, d = 20 nm, N = 3). This blue-shift is due to the deeper carrier confining potential of the dots at higher values of y. In contrast, with decreasing d and/or increasing N, the PL red-shifts from 1.1 µm (sample A: y = 0, d = 20 nm, N = 3) to ~1.3 µm, (sample C: y = 0, d = 1.7 nm, N = 10), evidence for electronic coupling between vertically stacked QDs. Therefore by engineering the carrier potential profile of the dots it is possible to cover a broad energy range for the room temperature light emission of QDs. This is of particular interest for extending the optical emission range of QDs to 1.3 µm, the window for signal transmission through silica fibers. Samples shown in Figure 4 exhibit a different thermal behaviour. In fact the thermal stability of the dot emission is strongly dependent on the composition of the matrix incorporating the dots. We found that the dots embedded in a (AlGa)As barrier and/or in a GaAs/(AlGa)As QW exhibit the highest thermal stability. This can be attributed to the low level of thermal escape of carriers from the dots towards the high energy levels of the AlGaAs barrier or the GaAs/(AlGa)As QW, which therefore act to prevent carrier depopulation of the dot levels. QUANTUM DOT LASERS QDs find significant interest especially for application in laser diodes. It is worth noting that the device, which benefited most from the introduction of quantum wells (QW), is the injection laser. The QW laser reached mass production within very few years because of its low cost, high performance and high reliability. QDs are believed to provide a promising way for a new generation of optical light sources such as injection lasers. QD lasers are expected to have superior properties with respect to conventional QW lasers. Theoretical predictions [7] of the intrinsic properties of QD lasers include higher characteristic temperature T o of threshold current, higher modulation bandwidth, lower threshold currents and narrower linewidth (Figure 5). - 3 -
The principal advantage of using size-quantized heterostructures in lasers originates from the increase of the density of states for charge carriers near the band-edges (Figure 1). When used as one active medium of a laser, this results in the concentration of most of the injected nonequilibrium carriers in an increasingly narrow energy range near the bottom of the conduction band and/or top of the valence band. This enhances the maximum material gain and reduces the influence of temperature on the device performance. Figure 6 shows the development of semiconductor diode lasers in terms of threshold current density as a function of time for various heterostructures based on double heterostructure (DHS), QWs and QDs. The recent developed QD lasers [8] based on InAs QD system have already showed record low threshold current density of 24 A/cm 2 at room temperature for a wavelength of 1.28 µm. This is almost a factor of two lower than what has been achieved for QW lasers [9]. Recently, InAs QD laser diodes with high light output power of 4.7W at 1.135 µm were reported [10]. Red light emitting QD lasers have also been successfully fabricated with AlInAs/AlGaAs [11] QD, and InP/GaInP [12] QD. It is possible to access new energies by combining materials with different lattice constants and energy gaps. Currently, the spectral range of III V semiconductor QD lasers extends from near infrared (1.84 µm for InAs (In,Ga,Al)As QD lasers on InP substrates [13] to the visible red range [11]. OTHER APPLICATIONS OF QDS Methods of detection of infrared (IR) radiation have been investigated for almost 200 years since the astronomer William Herschel discovered what is now called the infrared portion of the spectrum in 1800. Infrared detectors have been key components in thermal imaging, guidance, reconnaissance and communication systems. All important applications of infrared techniques, both for military and civil purposes (sciences, meteorology, medicine, industry, etc) rely on the detection of radiation in the 1-3 µm, 3-5 µm and 8-14 µm spectral range (the so-called atmospheric windows). The 8-14 µm wavelength region is especially important for imaging since the temperature of the human and environments bodies is around 300K corresponding to a peak wavelength of thermal radiation of about 10 µm. The materials, which cover the above wavelength regions, include II-VI, III-V and IV-VI compound semiconductors. With respect to the well-known HgCdTe detectors, GaAs/AlGaAs quantum wells devices have a number of potential advantages including the use of standard manufacturing technology based on advanced GaAs growth and processing techniques, highly uniform and well controlled MBE growth on large GaAs wafers, high yield, greater thermal stability, and intrinsic radiation hardness. Recently, there has been significant interest in developing novel QD infrared photodetectors (QDIPs). There are two major potential advantages of quantum dots over quantum wells as photodetectors [14], namely: (1) intersubband absorption may be allowed at normal incidence. In quantum well infrared photodetectors (QWIPs) only transitions polarized perpendicular to the growth direction are allowed, due to absorption selection rules. The selection rules in QDIPs are inherently different, and normal incidence absorption is, indeed, observed, (2) thermal generation of electrons is signicantly reduced due to the energy quantization in all three dimensions. Generation by LO phonons is prohibited unless the gap between the discrete energy levels equals exactly to that of the phonon. This prohibition does not apply to quantum wells, since the levels are quantized only in the growth direction and a continuum exists in the other two. Hence thermal-generation or recombination by LO phonons results, with a capture time of few picoseconds. Thus, it is expected that the signal-to-noise ratio in QDIPs will be significantly larger than that of QWIPs. The operation of photovoltaic QDIP fabricated from (InGa)As/GaAs heterostructures have been demonstrated by D.Pan et al [15]. These detectors are sensitive to normal incidence light. At zero bias and low temperature of 78 K, peak detectivity of 2x10 8 cm.hz 1/2 /W, with a responsivity of 1 ma/w at a wavelength of 13 µm, was obtained. The authors claim that this is the higest detectivity achieved for a QDIP operating in the photovoltaic mode, and device might be attractive for focal plane array imaging applications. As reported above QDs can be used to make better optical devices. They can also be used to make devices with a totally new functionality. Another application is an optical data storage medium, in which bits of information are stored as a single or few electrons within the dots. These memory devices based - 4 -
on QDs should require a very low switching energy because the information is stored as a single or few electrons. Optical illumination can be used to write the charge to be stored in the QDs. It is predicted that because each QD would carry a bit of information, the memory devices could potentially achieve ultra dense storage capacities of Terabit/in 2. Y.Sugiyama et al [16] at Fujitsu Laboratories, Japan, observed memory effect of InAs QDs buried in Schottky barrier diode with memory retention time of 0.48ms for the first time. The band diagram of their structure which contained a single layer of QDs is shown in Figure 7. Two sequential laser pulses with an interval time t irradiated the diode. Electron-hole pairs are generated in the InAs QDs after a pulse irradiation from the first laser. The electron escape from the QDs by tunnelling or thermo-ionic emission, and the holes stay in the QD. The residual holes decrease the photocurrent when the second laser pulse is irradiated. The retention time of the optical memory was determined from the interval time, t, dependence of the photocurrent difference I write and I read. These are preliminary results. However, this group believe that there is a possibility of using QDs for high density optical memory. Some devices in which electrons are transported in the vertical direction have shown a clear memory effect [17] of InAs QDs. Very recently, H.Son et al [18] have demonstrated the memory operation of InAs QDs in a lateral heterojunction field effect transistor (FET) structure. Lateral transport occurred through the parallel conduction of the two-dimensional channel and QD layers. This channel, which is induced by gate bias, is strongly influenced by the charge stored in InAs QD layers. This was shown by the current hysteresis curve and the capacitance-voltage measurements. The authors believe that the memory operation is due to the charge trapping effect of InAs QDs. QD memory devices using optical illumination to store charge in the QDs require a sensitive method of detecting the trapped photo-excited charge within the QDs. A.J.Shields et al [19] demonstrated that using a transistor structure (Figure 8) it is possible to detect the presence of single photo-excited carrier in a single QD. Conventional semiconductor single-photon detectors rely upon the avalanche process. However, in the device of Shields et al the gain derives from the fact that the conductivity of the FET channel is very sensitive to the photoexcited charge trapped in the QDs. Their FET contains a layer of InAs QDs adjacent to the channel and separated from it by a thin AlGaAs barrier. The capture of a single photo-excited carrier by a QD leads to a sizeable change in the source-drain current through the transistor, allowing the detection of a single photon. CONCLUSION In this article I have described progress made on some QDs devices. Other work on QD structures is progressing in many laboratories worldwide. Some of the studies cited in this article, like many reported in international journals, are still in their infancy, and many challenges remain in the development of high-performance QD devices for optical and electronic applications. For the case of QD lasers it is believed that they are ready for practical applications and their future is extremely promising. However, the replacement of QW lasers with QD lasers depends on the industry and its willingness to switch to new technology. If recent history of DHS and QW lasers is any guide, it is likely that QDs will lead to entirely new classes of materials and devices. - 5 -
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Figure 1: Schematic diagram of the density of states (DOS) in the conduction band (CB) and valence band (VB) for a (a) double heterostructure, (b) quantum well, (c) quantum wire and (d) quantum box laser. - 7 -
Figure 2: Scanning tunnelling microscope pictures (100x100 nm) of InAs/GaAs QDs grown by MBE on (100), (311)A and (311)B GaAs substrates [1] As can be seen, using substrates with different orientation can control the shape of the QDs. - 8 -
Figure 3: Schematic diagram showing ordering of quantum dots due to strain fields effects to form (a) vertical alignment, and (b) lateral alignment on patterned substrates. - 9 -
PL Intensity (arb. units) Al y Ga 1-y As InAs B GaAs A GaAs C RT 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 λ (µm) Figure 4: Room temperature (T = 290 K) PL spectra of samples A (InAs/GaAs QDs, d = 20 nm), B (InAs/Al 0.8 Ga 0.2 As QDs, d = 20 nm) and C (vertically stacked InAs/GaAs QDs, d = 1.7 nm and N = 10). The inset sketches the structure for the three samples. - 10 -
Figure 5: Theoretical predictions of the threshold current versus temperature in double heterostructure (DHS), quantum well (QWs), quantum wire and quantum box (QDs) laser. The threshold current as a function of temperature T is given by: J th (T)= J th (0).exp (T/T o ) where T o is the laser characteristic. - 11 -
Figure 6: Development of semiconductor diode lasers based on DHS, QWs and QDs [20]. The lowest current density achieved up to now is 6A/cm 2 [21]. Courtesy of D.Bimberg, Technical University of Berlin - 12 -
Figure 7: Device showing memory effect of InAs quantum dots. - 13 -
Figure 8: Cross-sectional view of a QD FET structure with a small area Schottky gate and scanning electron microscope image of the gate region for an FET with a 2 µm wide mesa and 4 µm long gate (Courtesy of Dr A.J.Shields, Toshiba Research Europe Ltd). A positive gate bias charges the QDs with electrons, which limits the mobility of the adjacent electron channel. Single photons liberate a trapped electron via capture of a photoexcited hole. This results in a detectable increase in the conductance of the electron channel. - 14 -